Wood-polymer composites of this category is based on polymerization of a monomer in the cavities of the cells (in-situ). The polymer produced is usually a thermoplastic (linear polymer). Monomers which belong to the vinyl monomer group are the ones usually used for this purpose. The vinyl monomers used are polymerized by means of radical chain polymerization. The vinyl monomers are nonpolar, and will therefore not swell or react with the cell wall of the wood material. Some persons skilled in the art therefore claim that wood-polymer materials of this type are not <<real>> composites because there is no chemical bonding between the two combined materials. However, there is a high extent of infiltration and the material acquires altered physical and mechanical properties, and hence should be defined as a novel type of material. Properties which are substantially increased are density, hardness, wear resistance and elastic modulus. Liquid water and water vapor movements in the new material are greatly reduced, especially along the grain. In wood, transport along the grain is many times greater than across the grain, but in the new material they are about equal. Because the polymer does not react with the structure of the cell walls, the basic properties of the woody cell wall are not changed. Because the pores of the wood are blocked by polymer, the moisture can only move within the unchanged woody cell wall, in spaces between the cell wall and the polymer in the lumen, or in any cracks that might be in the material. The time for humidification or drying of cell lumen wood-polymer composites will therefore be 10 to 20 times longer than for untreated wood. This delay is an advantage in changing environments (especially a spill of liquid water). Because of the longer time, and because the polymer may induce a mechanical restraint of swelling, the dimensional stability of the new material is slightly improved. However, this can lead to local stresses in the material, which can result in microcracks when the material is subjected to extreme moisture gradients.
The monomers usually utilized for cell lumen wood-polymer composite are methyl methacrylate and styrene and unsaturated polyester oligomers, because of large supply of these chemicals and hence their low cost, as well as the relatively simple method of polymerizing them. Negative aspects of methyl methacrylate monomer are its subtantial shrinkage (up to 25%) during polymerization and relatively high vapor pressure (it evaporates easily). Negative aspects of styrene monomer are its high vapor pressure (it evaporates easily) and easily-detectable odour (low odour threshold). A negative aspect of polyester oligomer is its higher viscosity which limits impregnation possibilities.
Monomer evaporation can lead to a low filling of surfaces by monomer. Monomer shrinkage can lead to substantail shrinkage and warping of materials during cure, and can cause voids between polymer and cell wall. By selecting different types of monomers these problems may be reduced.
One problem of previously known wood-polymer composites of this category is that after impregnation and curing, they continue to liberate vinyl monomers which have an easily-detected odour and which can be irritating and injurious to health.
Other problems of previously known wood-polymer composites are cracking, warping and colour alteration of the materials. These indicate that temperatures of the curing phase caused by exothermic heat of reaction have been too high, resulting in excess shrinkage and pyrolysis damage to the wood. The problem is increased dramatically with increased cross-sectional size, and in the past has limited the size of material which could be successfully treated.
Another problem of previously known wood-polymer composites results from a combination of moisture in the wood and the curing heat. Because of the exothermic heat of reaction, the temperature, especially of larger cross-sections, exceeds the boiling point of water. This causes rapid drying, followed by warping and cracking in moist wood. For this reason, previous art successful treatment has been generally limited to dry (6% moisture content and less) wood.